The present invention relates to oxygen sensors and more particularly to an oxygen sensor for measuring the oxygen content of a gas containing relatively high levels of oxygen. The invention is used primarily in connection with instruments for measuring the permeability of films and membranes, wherein oxygen is passed into a chamber, one wall of which is enclosed by a material membrane, and a second chamber on the other side of the membrane is coupled to the sensor. Oxygen which permeates through the membrane is detected by the sensor, which generates an electrical signal proportional to the quantity of oxygen detected.
An oxygen sensor of the type generally related to the present invention is disclosed in U.S. Pat. No. 3,223,597, issued Dec. 14, 1965, to Hersch. The Hersch patent discloses a general construction of an oxygen sensor and shows a plurality of layers of materials which are or may be utilized to construct a workable sensor. The principles of the Hersch invention are further elaborated in a construction disclosed in U.S. Pat. No. 4,085,024, issued Apr. 18, 1978, to Lawson. The Lawson patent discloses a particular construction and method of making the oxygen sensor, utilizing many of the same materials which are the subject of the present invention. A further construction of an oxygen sensor of the general type is shown in U.S. Pat. Nos. 5,139,638, issued Aug. 18, 1992, and 5,184,392, issued Feb. 9, 1993, and owned by the assignee of the present invention. These patents disclose improvements in construction which are utilized in connection with the present invention; particularly, improvements to the manufacturing steps which assure a uniform and continuous surface area contact between a cathode and anode via an electrolyte-retentive material wrapped around the anode.
Sensors of the above-described type generally utilize nickel cadmium (Ni-Cd) cathode elements in order to provide a highly sensitive device which can detect oxygen at levels down to parts per trillion. The device is essentially a Ni-Cd battery which is constructed in a manner so that oxygen levels determine the output current. For every oxygen molecule four electrons are released in a Faraday-type electrochemical reaction. As such, the electrodes are immersed in an electrolyte and a gas is passed through the device wherein the current that is generated by the electrodes is equal to the charge-per-second generated by the release of the four electrons per oxygen molecule.
In normal usage, this sensor is used to detect low oxygen contents in dry gas streams at flow rates of 5-50 cubic centimeters per minute (cc/min). The lifetime of the sensor is affected by the gas stream, which tends to dry the electrode as well as dissipate the original charge in the device. At a flow rate of approximately 20 cc/min, a typical sensor of the foregoing type dries out in about 300-400 hours of usage. This drying-out problem was addressed in U.S. Pat. No. 5,139,638 which added a humidifying element to the sensor construction.
The normal charge on sensors of this general type is 800-1,000 milliamp-hours (ma-hrs), at oxygen levels of up to 210 parts per million; at lower oxygen levels the charge has a lifetime in the range of 2,000-20,000 hours. Stated in percentage, this represents the normal lifetime range of a sensor detecting oxygen concentrations of about 0.02 percent. As the percentage oxygen concentration increases, the charge dissipates rapidly; for example, at a 1 percent oxygen concentration and a flow rate of 20 cc/min, the charge lifetime decreases to less than 20 hours. It is, therefore, apparent that sensors of the general type disclosed in the prior art are not suitable for detecting high oxygen concentration levels for any reasonable time period.
A number of modern barrier materials used in the packaging industry are designed to intentionally pass relatively high oxygen concentration levels. Such barriers are useful in packaging fresh fruits and vegetables and other products having a need to "breathe" during the time the product is in transition from packaging to consumption. These new breathable barrier materials permit an extended shelf life for such products before product quality is adversely affected.